1. Field of the Invention
The invention relates to atomic force microscopes and methods of operating atomic force microscopes.
2. Discussion of the Related Art
An atomic force microscope (AFM) is a high-resolution device for producing scanned images of the surfaces of samples. One type of AFM has a flexible cantilever with a narrow tip, a deflection detector, a sample stage, and cantilever and sample stage feedback/drive systems. The AFM produces an image by mechanically scanning the cantilever's tip over a surface of a sample. During scanning, the deflection detector measures the amount of cantilever bending. The amount of bending is related to the vertical force that the sample exerts on the cantilever's tip, i.e., the sample-cantilever force. During the scanning, the feedback/drive systems produce movements for restoring this sample-cantilever force to an initial state. The AFM produces a topographical image of the sample's surface by recording the tip displacement needed to keep the sample-cantilever force at a preselected setpoint value during scanning.
In some AFMs, the feedback system has an active electronic cantilever controller and a active electronic sample stage controller in a nested configuration. In the nested configuration, the cantilever controller responds to an error signal from the deflection detector. The error signal is substantially proportional to the difference between the force exerted by the sample on the tip and a preselected setpoint value. Thus, the error signal is roughly proportional to the amount of bending of the cantilever. In response to the error voltage, the cantilever controller generates an about proportional voltage to drive a piezoelectric device to produce a restoring displacement of the cantilever. In the nested feedback system, the sample stage controller receives the drive voltage produced by the cantilever controller. In response to the drive voltage, the sample stage controller generates a voltage to drive another piezoelectric device to produce a restoring displacement of the sample stage. Together, the restoring displacements of the cantilever and sample stage restore the force exerted by the sample's surface on the tip to the preselected setpoint value.
Many AFMs produce an image of a sample surface in which the resolution vertical to the sample's surface is very high, e.g., better than 1 nanometer (nm) or even better than 0.1 nm. To obtain such high resolutions, setups for AFMs typically have to limit environmental vibratory noise. One source of such noise is low-frequency oscillations of the floor in the room where the AFM is set up, i.e., building noise. Usually, a setup for an AFM includes mechanical isolation to damp building noise and an acoustic box to eliminate acoustical noise. In many cases, environmental noise limits the upper scanning speed and/or the depth resolution obtainable with an AFM.